Catalytic reforming is one of the basic petroleum refining processes for upgrading light hydrocarbon feedstocks, frequently referred to as naphtha feedstocks. Products from catalytic reforming can include high octane gasoline useful as automobile fuel, aromatics (for example benzene, toluene, xylenes and ethylbenzene), and/or hydrogen. Reactions typically involved in catalytic reforming include dehydrocylization, isomerization and dehydrogenation of naphtha range hydrocarbons, with dehydrocyclization and dehydrogenation of linear and slightly branched alkanes and dehydrogenation of cycloparaffins leading to the production of aromatics. Dealkylation and hydrocracking are generally undesirable due to the low value of the resulting light hydrocarbon products.
Catalysts commonly used in commercial reforming reactions often include a Group VIII metal, such as platinum or palladium, or a Group VIII metal plus a second catalytic metal, which acts as a promoter. Examples of metals useful as promoters include rhenium, tin, tungsten, germanium, cobalt, nickel, rhodium, ruthenium, iridium or combinations thereof. The catalytic metal or metals may be dispersed on a support such as alumina, silica, or silica-alumina. Typically, a halogen such as chlorine is incorporated on the support to add acid functionality. In addition to Group VIII metals, other reforming catalysts include aluminosilicate zeolite catalysts. For example, U.S. Pat. Nos. 3,761,389, 3,756,942 and 3,760,024 teach aromatization of a hydrocarbon fraction with a ZSM-5 type zeolite catalyst. U.S. Pat. No. 4,927,525 discloses catalytic reforming processes with beta zeolite catalysts containing a noble metal and an alkali metal. Other reforming catalysts include other molecular sieves such as borosilicates and silicoaluminophosphates, layered crystalline clay-type phyllosilicates, and amorphous clays.
In addition to selection of catalysts for reforming, various processes for reforming a naphtha feedstock in one or more process steps to produce higher value reformate products are known in the art. U.S. Pat. No. 3,415,737 teaches a process for reforming naphtha under conventional mild reforming conditions with a platinum-rhenium-chloride reforming catalyst to increase the aromatics content and octane number of the naphtha. In U.S. Pat. No. 3,770,614 there is disclosed a process in which a reformate is fractionated and the light reformate fraction (C6 fraction) passed over a ZSM-5-type zeolite to increase aromatic content of the product. U.S. Pat. No. 3,950,241 discloses a process for upgrading naphtha by separating it into low- and high-boiling fractions, reforming the low-boiling fraction, combining the high-boiling naphtha with the reformate, and contacting the combined fractions with a ZSM-5-type catalyst. U.S. Pat. No. 4,181,599 discloses a process for reforming naphtha comprising separating the naphtha into heavy and light fractions and reforming and isomerizing the naphtha fractions. U.S. Pat. No. 4,190,519 teaches a process for upgrading a naphtha-boiling-range hydrocarbon which comprises separating the naphtha feedstock into a light naphtha fraction containing C6 paraffins and lower-boiling hydrocarbons and a heavy naphtha fraction containing higher-boiling hydrocarbons, reforming the heavy naphtha fraction and passing at least a portion of the reformate together with the light naphtha fraction over a zeolite catalyst to produce an aromatics-enriched effluent. Different catalysts may be employed in different process steps during the reforming of naphtha feedstocks as described in U.S. Pat. No. 4,627,909, U.S. Pat. No. 4,443,326, U.S. Pat. No. 4,764,267, U.S. Pat. No. 5,073,250, U.S. Pat. No. 5,169,813, U.S. Pat. No. 5,171,691, U.S. Pat. No. 5,182,012, U.S. Pat. No. 5,358,631, U.S. Pat. No. 5,376,259 and U.S. Pat. No. 5,407,558, for example.
Even with the advances in naphtha reforming catalysts and processes, a need still exists to develop new and improved reforming methods to provide higher liquid yield, improve hydrogen production, and minimize the formation of less valuable low molecule weight (C1-C4) products. It has been discovered that interstage feed separation in a staged reforming process and lower pressure in the final stage of a multistage reforming process can improve the RON (Research Octane Number), aromatics content, C5+ liquid yield, hydrogen production, and catalyst life.